To adhere to the legal regulations concerning the maintenance of clean air, modern motor vehicles operated with internal combustion engines are generally equipped with exhaust-gas purification systems. Said systems have for example so-called oxidation catalytic converters and, in particular in diesel combustion engines, also particle filters for absorbing soot particles in the exhaust gas. In order to prevent blockage of the particle filter, it may be necessary in the case of a corresponding loading of the particle filter for a regeneration process to be realized, in which process the particles which have accumulated in the particle filter are generally burned in the particle filter as a result of a corresponding increase in temperature of the exhaust gas to temperatures between 500° C. and 700° C. The increase in temperature is in this case may be realized by targeted variation of the operating parameters of the internal combustion engine.
As is described for example in DE 10 2011 014 718 A1 and also in US 2010 01 07 737, accumulation of sulfur-containing compounds in the particle filter and, if appropriate, also in the oxidation catalytic converter occurs at the same time, in particular during the operation of the respective internal combustion engine with fuels having increased sulfur content. In the regeneration phase of the particle filter, a rapid release of the accumulated sulfur compounds, which, together with the steam present in the exhaust gas, can form sulfuric acid, then occurs owing to the greatly increased exhaust-gas temperatures. As a result of the exhaust-gas flow being cooled again on the path through the exhaust-gas system to temperatures below the acid dew point, an aerosol, which is visible as dense white smoke, so-called “white smoke”, is formed.
The document DE 102011014718 B4 discloses a method for avoiding white smoke. Accordingly, the desorption, that is to say the release of the sulfur compounds, is realized in a temperature range of 300° C. to 500° C., e.g. of 400° C. to 450° C. At these exhaust-gas temperatures increased in this manner, the desorption of the sulfur compounds can over a longer time period, for example up to 10 minutes. The concentration of the aerosol in the exhaust gas is thereby kept so low that no white smoke can be perceived. This controlled desorption is in each case carried out before the regeneration of the particle filter. The further increase in the exhaust-gas temperature for the regeneration of the particle filter is realized only afterward. However, in this method, the desorption of the sulfur compounds has to be carried out over an extended time period prior to each regeneration of the particle filter owing to the size of the loading of the particle filter with sulfur compounds being unknown. This leads to increased fuel consumption, even during the operation of the internal combustion engine with fuels having low sulfur content where it might not be necessary to carry out the above-described method.
An attempt is made to solve said problem by way of the subject matter disclosed in the document DE 10 2009 058 107 A1. Here, it is established by means of a determination means whether the temperature of the particle filter falls below a predefined threshold value, for example a temperature of 340° C. Since sulfuric acid, as an example of a sulfur-containing compound, decomposes only above a temperature of around 340° C., in the case of said threshold value of the temperature being fallen below, it is assumed that sulfur-containing compounds are accumulating in the particle filter.
The accumulated quantity of sulfur-containing compounds in the particle filter is determined on the basis of the quantity of fuel injected into the internal combustion engine and a value, predetermined on a country-specific basis, for the sulfur content of the fuel. Here, as soon as the temperature in the particle filter falls below a threshold value, the quantity of sulfur-containing compounds accumulating in the particle filter is added up. With the reaching of a predetermined threshold value, the desorption is then started by increasing the exhaust-gas temperature to a value of approximately 350° C. During the desorption, the previously accumulated quantity of sulfur is reduced by computational means, and the desorption is ended as soon as the computationally determined quantity of sulfur reaches the value zero or it is established by means of a sensor that sulfur compounds no longer occur in the exhaust gas. In this method, a sulfur content, known on a country-specific basis, of the fuel is assumed. It is furthermore proposed that, if a sensor for detecting the sulfur compounds in the exhaust gas is provided, it is possible on the basis of the sensor data for a different sulfur concentration in the fuel to be inferred and for the corresponding presetting value to be corrected.
Owing to the imprecise knowledge of the sulfur content and possibly further variables influencing the accumulation of sulfur and desorption, the method however appears to be imprecise and susceptible to errors. An additional sulfur sensor for detecting the sulfur content in the fuel, on the basis of which sensor the loading of the particle filter could be determined more accurately and it would be possible to carry out the desorption in a more targeted manner, could remedy the situation here. However, such a sensor is too expensive in many application cases and therefore cannot be used.